Ring laser gyros (RLG) have been used as inertial rotation sensors for several years. A typical RLG may include three or more mirrors arranged in a triangular or square configuration for reflecting a laser beam causing the laser beam to rotate. First and second laser beams propagate in opposite directions (clockwise and counterclockwise). The RLG has a nominally zero beat frequency between the two beams when the RLG is not rotating. Rotation of the RLG about its input axis causes the effective path length to increase for the beam traveling in the direction of rotation, and to decrease for the beam traveling opposite to the direction of rotation. A beat frequency representative of the frequency difference between the beams results, which is indicative of the angular velocity of the RLG.
A major problem in ring-laser gyros is the phenomenon of lock-in. Lock-in occurs if the input angular velocity falls below a critical value, known as the lock-in rate, .OMEGA..sub.L, causing the gyro to cease providing a useful output because the frequencies of the two oppositely directed travelling waves (E.sub.1, E.sub.2) in the cavity lock together. A typical known method for overcoming the effect of lock-in is to "dither" the gyro. The dither is an auxiliary input angular velocity, which is applied to the gyro in addition to the external angular velocity that the gyro is required to measure. The dither angular velocity is a periodic or almost periodic signal having an average value of zero. The amplitude of the dither input is selected so that the sum of the external angular velocity and the dither velocity are well above the lock-in rate during most of the dither cycle.
The most common technique of supplying the dither angular velocity in the current generation of ring-laser gyros is called "body dither" whereby the gyro block is mounted on a torsional spring assembly so that it is free to move angularly relative to the case of the instrument. The block is set into oscillation, the amplitude of which is maintained by means of an automatic gain control (AGC). However, body dither is undesirable in that the spring assembly requires power to keep it in motion, occupies excess space, and adds unwanted weight. Moreover, the instantaneous angular motion of the gyro block may be too large to permit mounting thereon of the accelerometers required for an inertial navigation system. In addition, the injection of mechanical noise into the dither motion, necessary for the elimination of lock-in, leads to noise, known as random walk, thereby limiting the accuracy of the RLG. Also, the motion of the gyro block may induce an effect known as "coning" in which the input axis of the instrument traces out a cone in space which may require the use of high speed compensating computations to determine the true angular velocity.
Many attempts have been made in the prior art to overcome lock-in in RLG systems. A number of such prior systems will now be briefly described.
In Killpatrick, U.S. Pat. No. 3,323,650, a bias system for a laser gyroscope is shown for preventing lock-in of the two beams at low rotational rates when the frequency difference between the beams is small. The laser gyroscope is mechanically oscillated with respect to the base of the laser gyroscope, to cause the relative rotation rate between the beams to be higher than that at which lock-in occurs. The gyroscope is oscillated about its input axis for maintaining an input rate higher than the lock-in rate. Another feature disclosed is the use of optical means for preventing lock-in, wherein a Faraday medium is used to bias the frequency of the two laser beams. Accordingly, through either mechanical or optical biasing, the frequency difference between the counter rotating laser beams is maintained for a majority of the time for avoiding lock-in of the beams.
Hutchings et al., U.S. Pat. No. 3,752,586, teaches a method and apparatus for minimizing frequency locking of a ring laser gyroscope at low rotation rates. The counter rotating beams are frequency modulated in phase opposition to one another with a plurality of signals with mutually different frequencies, and with each of the signals having a predetermined modulation index, whereby the modulation indices of the signals are adjusted for minimizing frequency locking.
Dorsman, U.S. Pat. No. 4,099,876, teaches in a ring-laser gyro the use of a dual phase-coded controller for controlling a three-terminal duo-mode bimorph device having a translational control mode terminal and a torsional control mode terminal pair. The controller includes two mutually phase-coded and selectively biased AC reference signal generators, and a duo-mode differential amplifier interconnecting the signal generators to the bimorph device for dithering each mode of the duo-mode bimorph in response to a mutually exclusive one of the reference signals. The amplifier further includes a control input coupled to one of the two sources of a bipolar control signal for control of a mutually exclusive mode of the duo-mode bimorph device. The control system is a closed loop control system for insuring constant correction of any errors occurring in the system.
Hutchings U.S. Pat. No. 4,281,930 shows a ring laser gyro in which piezoelectric elements are used to mechanically dither each mirror in a phase relationship of 360.degree./number of mirrors. The mirrors are also oscillated at the same frequency. In this manner, lock-in is substantially prevented. Hutchings et al. U.S. Pat. No. 4,422,762 and Egli U.S. Pat. No. 4,592,656 also slow ring laser gyros with dither mechanisms.
Curby et al., U.S. Pat. No. 4,597,667, discloses an apparatus and method for providing dither control for a multiplicity of ring laser gyros to minimize lock-in and coning errors. The ring laser angular rotation sensors are dithered in an intermittent manner such that a plurality of such sensors having substantially identical natural dither frequencies are periodically energized for dithering in response to the amplitude of dither for a particular sensor, for minimizing lock-in. The use of mechanical dithering is disclosed for preventing lock-in. Electrical dithering is accomplished through the use of a Faraday cell in this latter embodiment. The driving voltage is turned on and off, whereby turn on is accomplished when the current decreases below a predetermined amplitude, and turned off when the current increases above a predetermined amplitude. In this manner, the lock-in band of each ring laser rotation sensor is reduced. Coning effects of low rotation rates or frequencies between three-ring laser angular rotation sensors are avoided through use of mechanical dithering. In an alternative embodiment, lock-in is also avoided by frequency or phase modulating the bias drive voltages in a Faraday cell.
Stjern et al., U.S. Pat. No. 4,653,918, teaches the use of low-Q assemblies for a plurality of laser gyro-dither motors. Drive signals of randomly varying frequency are utilized for dithering the gyros. The frequency modulated dither varies the lock-in rate of the gyro to reduce lock-in effects. The modulating system includes a sine wave generator and a generator for producing a periodic signal having a frequency different from the frequency of the sine wave, whereby the sine wave and the periodic signal are combined for providing the drive signal, with the dither means being responsive to the drive signal for imparting dither motion to the gyroscope.
Wirt, U.S. Pat. No. 4,779,985, discloses a dither suspension for a ring laser gyroscope. A mechanical structure for the dither suspension is disclosed.
Sewell, U.S. Pat. No. 4,790,658, teaches a system for obtaining a correction signal in a dithered ring laser gyroscope, whereby the correction signal is stripped of any components of the dither signal. In this manner, errors due to the dither signal and the control signal supplied by the ring laser gyro are substantially eliminated through such removal of the dither component.
Benoist, U.S. Pat. No. 4,801,206, teaches a ring laser gyroscope system. In column 3, lines 28-32, it is indicated that "one approach to reducing lock-in error is to superimpose a random signal upon the amplitude of the dither driving amplifier. However, the superposition of a random signal on the dither driver produces other substantial errors." It is recognized that the natural resonant frequency of the dither flexure varies with temperature, and that it is desirable to drive the dither flexure at its natural resonant frequency in order to make the most efficient use of the dither energy. The use of a dither drive controller monitoring the resonant frequency of the dither flexure at any given time, and thereby adjusting the dither frequency in order to maintain the frequency of the dither signal at the resonant frequency for optimizing the dithering function, is described. A hinge is required to "dither" the mirrors.
Hoo, U.S. Pat. No. 4,807,999, teaches a ring laser gyroscope that includes two laser sources and a common ring resonator. Two frequency tracking servos are included for peaking the intensity of the clockwise and counter clockwise propagating light beams. A first dither signal generator is used in conjunction with the first laser source, and a second dither signal generator is used in association with the second laser source for providing dithering.
In Lim et al., U.S. Pat. No. 4,824,252, two independent feedback control systems are used for individually altering the lasing path of the ring laser angular rate sensor for minimizing lock-in. One or more mirrors defining the lasing path are dithered and positioned by independent feedback control systems for providing two degrees of freedom for controlling the lasing path.
Benoist, U.S. Pat. No. 4,844,615, teaches a system for correcting random walk errors induced by rate reversals in a dithered ring laser gyroscope. In column 3, lines 28-30, it is indicated that "typical practice is to superimpose a random signal upon the amplitude of the dither driving amplifier". The system generates an accumulated correction in software for each transversal of the lock-in region of the ring laser gyroscope. Heterodyne signals and individual beam intensity signals are utilized for providing the accumulated correction information. The method of the invention includes the step of determining the magnitude of coupling between the counter rotating beams, and further includes detecting changes in the direction of the dither oscillations, measuring the intensities of the two beams, and forming a sum signal from the measured intensities. A Heterodyne signal is formed that is indicative of the beat frequency produced by interference between the counter rotating beams, whereby the sum signal is demodulated with the heterodyne signal. In this manner, the degree of coupling between the beams is determined.
Upton, Jr., U.S. Pat. No. 4,846,574, teaches the use of a retro-reflector external to the closed-loop path of a ring laser system for redirecting energy of one of the counter rotating beams into the direction of travel of the other counter rotating beam, for substantially reducing lock-in effects.
Sewell, U.S. Pat. No. 4,898,469, describes a system for removing a dither signal from the output signal of a dithered ring laser system.
To avoid the problems of body dither, Ljung et al., as described in U.S. Pat. Nos. 4,410,274 and 4,410,276, teaches a mirror dither concept called "Doppler Mirrors". In the Doppler mirror, instead of dithering the entire gyro body, only two of the mirrors on the RLG are dithered. As discussed in these patents, and in a technical paper entitled "Reduction of Beam Coupling In a Ring-laser Gyro by Doppler Shifting of The Scattered Light" by R. A. Patterson, B. Ljung, and D. A. Smith in SPIE, Vol. 487, pp. 78-84, 1984, the lock-in effect is minimized, or entirely canceled when the oscillatory motions of the two mirrors are exactly equal and opposite, and when the amplitude of the oscillation is at one of the first order Bessel nulls.
As shown in U.S. Pat. Nos. 4,410,274 and 4,410,276, the Doppler mirror concept was applied to a gyro with a triangular light path. The concept, however, is not limited to a triangular light path. The application to a gyro with a square light path is described in Martin U.S. Pat. No. 4,686,683, wherein it is shown that the effect described in the earlier patents by Ljung et al. is achieved by moving two adjacent mirrors of the four that determine the light path. The motion of each of the two moving mirrors must be equal in magnitude and opposite in sign to that of the other.
To achieve the proper Doppler mirror effect, it is essential that the motion of the two moving mirrors have exactly the correct amplitude, and be 180.degree. out-of-phase. The systems described in the earlier patents listed above, however, are open loop systems. In such systems, an external oscillator provides a reference signal which each of the mirrors is required to track, one with zero phase shift and the other with 180.degree. of phase shift. Although Martin's U.S. Pat. No. 4,686,683 teaches the use of a feedback control system to maintain the amplitude of the oscillation, no means is provided to maintain the desired phase relation. It has been shown that even a small deviation of phase from the ideal 180.degree. will result in excessive "residual lock-in", .OMEGA..sub.RL. Hence a gyro using the drive system described in U.S. Pat. No. 4,686,683 does not operate reliably.
Another problem with the use of an external oscillator as the source of the reference motion is that this motion may be difficult for the mirrors to achieve owing to their mechanical construction. Since the mirrors have a natural resonance frequency, they will move most readily at their frequency of resonance, which should thus be chosen as the frequency at which the mirrors should be vibrated to achieve the "Doppler Mirror" effect. However, the resonant frequency of the mirrors will not be constant because of temperature and other similar effects. Thus, with an external oscillator as a reference signal, it will not be possible to maintain the motion of the mirrors at resonance.